Passenger seats, and particularly aircraft passenger seats, are designed to ensure passenger safety in the event of a crash. For example, certain regulations require that seats comply with occupant protection/head impact tests. The general intent is that the seat has a frangible joint designed to fail during an impact, reducing head accelerations. However, the seats must also withstand typical non-crash load abuse, such as passengers leaning heavily against the seat, using the seat as a brace, and other non-crash pressures. Seat design and manufacture thus continues to be an engineering challenge, in both design and performance.
Typically, a seat back is attached to a stable quadrant arm of a seat frame. In order to meet head impact criteria testing, head impact loads are distributed from the seat back to shear pins on both sides of the seat back. The shear pins constrain rotation of the back relative to the quadrant arms until impact. At impact, the shear pins serve as a break over device, designed to fail during an impact event and to allow the back to rotate forward. This can reduce head accelerations. However, shear pins have strict limits on breakout force and timing because they must be strong enough to survive static loading and can only allow break over when impact loads exceed the ultimate load on both pins. The challenge is often that because the shear pins must withstand general abuse loads, they may be so strong as to require excessive acceleration in order to break/shear properly. However, if the strength of the frangible joint/shear pin is reduced, the seat may not be strong enough to withstand expected general abuse loads. For example, some of the current seat designs have problems during 10 degree impact events, when asymmetrical loading on the back requires high rigidity in the back structure to transfer sufficient loads to both shear pins. In general, a shear pin break over device necessitates a highly reinforced back structure, rigid enough to transmit loads to both quadrant arms, but also cushioned to reduce head accelerations on initial impact. Achieving this goal can add weight to the back structure, and can require costly iterations of testing. Improvements to break over devices are thus desirable.